Karl the Mechan
Posts: 304
Joined: 12/5/2002
From: Bodoe, NORWAY
Status: offline
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¡§Keeping up with two nitro-trucks, except lagging in the cornering and long straight runs¡¨
Umm, if you lag in the corners AND the straights, which part of the track did you keep up with the nitro¡¦s? I may be missing a point here, keeping in mind English is only my second language ƒº
Hi there James_dean5, I can understand the need for speed and that you¡¦ve gathered information about how to estimate the car¡¦s tempo. I choose to believe you¡¦ve been misguided in the understanding of velocity and forces involved. Let¡¦s go through the numbers step by step:
THE 17,6 MPH (28,1 Km/h) CALCULATION:
Motor revs are depending on load, and we have absolutely no way of telling the revs while driving. If the motor revs 18000 at idle, it will go slower when load is applied. But let¡¦s ASSUME your motor spins at 18000:
18000 (motor-rpm) / 7,96 (std gearing) = 2261,3 (wheel-rpm). The wheel spins 2261,3 rounds per minute.
Pi (3,14) x d (wheel diametre, 65mm) = 204 mm = 20,4 cm. The wheel travels 20,4 cm per revolution.
2261,3 (wheel-rpm) x 20,4 cm = 46124,4 (cm per minute) = 0,46124 (Km per min)
0,46124 (Km/min) x 60 (min per hour) = 27,67 Km/h (17,3Mph). THEORETICAL speed.
¡§Add 5% for the bearings ???¡¨
How do you make a motor spin faster if you add bearings on the gears and wheels? The motor doesn¡¦t know that...? If bearings are to make a motor rev faster, they should be placed on the motor-shaft. To replace the metal/bronze bushings.
In real life, the gearboxes and axles causes drag. Add bearings, and they cause LESS drag. Of course the car will go faster with bearings but that also means the motor will rev faster. In your 17,6 MPH calculation it would rev at 18313.
THE 23,6 (37,7 Km/h) MPH CALCULATION:
By ¡§upgraded tranny¡¨ I take it you mean the ¡§53342 TL01 speed-tuned gear set¡¨. I had no idea what gear-change it resulted in but thanks for telling me. Here we go:
18000 (motor-rpm) / 5,96 (new gearing) = 3020,1 (wheel-rpm).
Pi (3,14) x d (65mm) = 204 mm = 20,4 cm. The wheel still travels 20,4 cm per revolution.
3020,1 x 20,4 cm = 61610,7 (cm/min) = 0, 616107 (Km/min)
0, 616107 (Km/min) x 60 (min/hour) = 36,97 Km/h (23,1Mph). Again THEORETICAL speed.
LOAD
Motor revs are depending on load.
- Gearboxes/axles rolling resistance. Which increases with increasing revs.
- Wheel rolling resistance. Rubber compound ans inserts play a role here.
- Wind resistance. Cd x A. (Resistance coefficient x fronal area) Which increases with the square of velocity.
Add all these loads.
To confuse matters further, we must take into concideration the CENTRIFUGAL FORCES.
When a wheel rotates fast, it expands. Making its diametre larger than its original 65 mm. Therefore it will travel MORE than 20,4 cm per revolution. Meaning, the car will go faster. The faster the wheel rotates, the more it expands.
SUM UP
We can calculate revs and load all day long, it will only give us theoretical figures. Even a Tamiya speed-checker doesn¡¦t give anything but theory. To find out how fast the car really goes, we need to go out and test it with a spped-trap or a laser-gun. Simple as that.
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TL01 Setup - 
How did you measure that ? - 